Improve BUILD_TIMELESS

[pietrushnic]

line information in asserts

Absolute paths can be different and might be a problem, but line numbers seem harmless. The obvious fix is to replace absolute paths with relative ones when they appear. I think coreboot mostly uses relative ones, while vboot or some other components are build with absolute ones.

git commit hashes

If they can be specified explicitly during build, then one could have the same results when building from a patched tarball.

Originally posted by @SergiiDmytruk in #889 (comment)